Composite structures are used in a wide variety of applications due to their high strength-to-weight ratio, corrosion resistance, and other favorable properties. In aircraft construction, composites are used in increasing quantities to form the fuselage, wings, horizontal and vertical stabilizer, and other components. For example, the wing of an aircraft may be formed of composite skin panels co-cured or co-bonded to internal composite structures such as composite stringers and composite spars. The composite stringers and spars may extend along a spanwise direction from the wing root to the wing tip and may generally taper in thickness along the spanwise direction to gradually reduce the stiffness of the stringer or spar.
Composite stringers and spars may be provided in a variety of cross-sectional shapes. For example, a composite stringer may be formed in a hat-shaped cross section and referred to as a vent stringer or hat stringer. In another example, a composite stringer may be formed in a T-shaped cross section referred to as a blade stringer. A blade stringer may be formed by bonding together two L-shaped charges in back-to-back arrangement. Each one of the L-shaped charges may have a flange and a web interconnected by a radiused web-flange transition. When the webs of two L-shaped charges are joined back-to-back, a lengthwise notch or part cavity (e.g., a radius filler region) is formed between the opposing web-flange transitions. To improve the strength, stiffness, and durability of the stringer and the bond between the stringer and a skin panel, the part cavity is typically filled with a radius filler which may be referred to as a noodle and which is typically formed of composite material.
Composite radius fillers suffer from several drawbacks which detract from their overall utility. For example, certain radius fillers may exhibit reduced structural performance due to susceptibility to cracking which may correspond to a relatively low pull-off strength at the bond between the stringer and a skin panel to which the stringer is bonded. Furthermore, certain radius filler configurations have inside radii that vary along a lengthwise direction which may prevent non-destructive inspection (NDI) of the inside radii using acoustic inspection methods. In addition, certain radius filler configurations require the assembly of multiple components to form the radius filler, and which has an adverse impact on manufacturing cost and schedule. Furthermore, certain stringer configurations require a radius filler having an asymmetric shape which is difficult to manufacture using existing radius filler configurations.
As can be seen, there exists a need in the art for a radius filler that provides improved structural performance including reduced susceptibility to cracking, improved ability to tailor the stiffness characteristics, and improved pull-off strength. Furthermore, there exists a need in the art for a radius filler that improves the inspectability of the composite structure containing the radius filler, and which can also be manufactured in a low-cost and timely manner.